In semiconductor photolithography equipment, alignment is a critical subsystem because its accuracy tends to determine the overlay accuracy of the semiconductor photolithography equipment. Machine vision-based alignment systems have gained extensive use because of a range of advantages such as a simple structure, high speed and high precision.
A photolithography tool employs an objective lens for correlating reticle coordinates to wafer coordinates. However, due to the absence of such a high-precision optical member as the objective lens, a structurally simpler quasi-lithography apparatus has to rely on dedicated optical and control systems for dual-layer alignment.
US Pat. App. Pub. No. 2009/0251699 A1 discloses a dual-layer alignment solution in which a movable beam splitter is disposed between the dual-layer objects to be aligned with each other. This beam splitter is capable of imaging marks on both the top and bottom layers. A lens is carried by a motion stage to or from a position for alignment. However, in this solution, since the motion stage and the lens are independent of each other, the system is complex in terms of structure and motion control, which is unfavorable to alignment accuracy and reliability.